Proof of logarithmic properties.

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Homework Help Overview

The discussion revolves around proving two logarithmic properties: the first property relates to the expression ln(b^(1/n)) and its equivalence to (1/n)ln(b) for b>0, while the second property concerns the expression ln(a^r) equating to rln(a) for any rational number r and a>0. The original poster expresses confusion regarding the differences in proving these properties compared to previously established logarithmic identities.

Discussion Character

  • Conceptual clarification, Problem interpretation, Assumption checking

Approaches and Questions Raised

  • The original poster attempts to understand the distinction between proving logarithmic properties for natural numbers versus rational numbers. They question the relevance of setting b=an and express uncertainty about how this aids in the proof. Other participants suggest that the previous proof method may not apply to rational numbers and encourage exploring the implications of defining r as a fraction.

Discussion Status

Some participants have provided insights into the differences in proof requirements for natural versus rational numbers, noting that assumptions made in the natural number case may not hold for rational numbers. The original poster has made progress on part 1 but remains uncertain about part 2, indicating an ongoing exploration of the topic.

Contextual Notes

The original poster has previously proven ln(a^n) = nln(a) for natural numbers and is now tasked with extending this proof to rational numbers. There is a recognition of the need to adapt the approach due to the nature of rational numbers.

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Homework Statement



There are two log properties that I have to prove:
1) Explain why ln(b1/n)=(1/n)ln(b) for b>0, set b=an

2) Explain why ln(ar)=rln(a) for any r in Q and a>0, ie r is rational.

Homework Equations



ln(an)=nln(a)

The Attempt at a Solution



In a previous question I have already proved that ln(an)=nln(a), where n is a natural number. What I'm unsure about, is how is this any different? For 1), I'm not sure why you set b=an? Wouldn't you get ln((an)1/n) = ln(a)? I'm not sure how this helps me find the solution.

Similarly for 2), I'm unsure how it is any different to proving that ln(an)=nln(a).

Any help would be great!
 
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The obvious difference is that you proved a result m ln(a)=ln(a^m) when m is a natural number, and now they want you to do it when m is not a natural number. The proof that you used for this will not work when m is not a natural number (unless you did something really clever) because at some point you assume that it was, probably in order to write am as a*a*a...*a a multiplied together m times (which doesn't make sense if m=1/2 for example)

If b=an, what does ln(b) and ln(b1/n) become? Use it to do something cool
 
Office_Shredder said:
The obvious difference is that you proved a result m ln(a)=ln(a^m) when m is a natural number, and now they want you to do it when m is not a natural number. The proof that you used for this will not work when m is not a natural number (unless you did something really clever) because at some point you assume that it was, probably in order to write am as a*a*a...*a a multiplied together m times (which doesn't make sense if m=1/2 for example)

If b=an, what does ln(b) and ln(b1/n) become? Use it to do something cool

Thanks for your help. I worked out part 1, but I'm still unsure on part 2. For proving the law for a natural number I did:
Let y=ln(a), for a>0,
ey=a, and if we raise each side to the power n we get:
(ey)n=an
ln(ey*n)=ln(an)
y*n = ln(an), but y = ln(a)
n*ln(a) = ln(an)

So I'm still unsure how it's any different for any rational number?
Any help would be great :)
 
Use the definition of Q, namely that r =m/n, where m,n are in N.
 
Ah I get it now! Thanks :)
 

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